This is a design for a large cardboard parabolic
reflector that could be used to
focus sound onto a microphone. The method was presented in Scientific
American by Alex McEachern and
Paul Boon. I built a version of it in 1995 and mounted it on a base of
my own design.

The units were originally in inches but they can be
interpreted as any unit.

Radius

R=

27

No. of
Sections

N=

12

Focus

f=

12

a=

0.0208

Base of
Triangle

B=

17.37

Row

X

Y

Y1

Z

Vd

D

1

3

0.19

0.75

3.05

3.05

0.01

2

6

0.75

1.69

3.14

6.20

0.05

3

9

1.69

3.00

3.27

9.47

0.12

4

12

3.00

4.69

3.44

12.91

0.24

5

15

4.69

6.75

3.64

16.55

0.41

6

18

6.75

9.19

3.87

20.42

0.63

7

21

9.19

12.00

4.11

24.53

0.92

8

24

12.00

15.19

4.38

28.91

1.28

9

27

15.19

18.75

4.66

33.56

1.72

10

30

18.75

Steps in creating one section of the paraboloid.(54 inches wide with a
focal length of 12 inches)

Draw an isosceles triangle with a height of 33.56
inches and base of 17.37 inches

To determine how much to trim each side of the
triangle (dashed lines above) draw trim line through the following
points from each edge:
at 3.05 inches down from the apex plot a point 0.01 inches in from each
edge
at 6.02 inches down from the apex plot a point 0.05 inches from each
edge
at 9.47 inches down form the apex plot a point 0.12 inches from each
edge
etc.

Continue this until you reach row 9

Connect these plotted points with a straight edge

Cut out section along trim line and base.

Using first section as a pattern trace out 11 more
sections and cut them out

Construct paraboloid by gluing the edges of the
sectors together. I used hot glue.

A spreadsheet to modify the
above calculations that I
programmed before I built the dish.